The present invention relates to a multi-domain vertical-alignment liquid crystal display device which can ensure a wide viewing angle without decreasing the effective aperture ratio.
Presently available liquid crystal display devices generally use a TN (Twisted Nematic) mode as a liquid crystal display mode. In the TN mode, however, the viewing angle is narrow, and tone reversal occurs when the screen is viewed from an oblique direction, which seriously reduces the display quality. Moreover, the response speed is low in a halftone region and the outlines of video pictures are not clear. Such problems of the TN mode cannot be completely overcome even by phase compensation.
As the alternatives to the TN mode, various liquid crystal display modes, such as an IPS (In-Plane Switching) mode, an OCB (Optical Compensated Birefringence) mode, and a MVA (Multi-domain Vertical Alignment) mode, have recently been developed. Above all, the multi-domain vertical-alignment mode has received attention as a technique of increasing the viewing angle.
In the multi-domain vertical-alignment mode, a method using electric slits (SID 97 DIGEST p.461), a method using protuberances on the surface of a substrate (Japanese Patent No. 2947350), and the like have been proposed to carry out multi-domain alignment of the liquid crystal. Furthermore, in order to improve the transmittance of the liquid crystal display device of the multi-domain vertical-alignment mode and to overcome the problem of color shifting among the tones, methods for adding a chiral material and orienting the liquid crystal molecules in a twisted manner while a voltage is applied have been proposed (as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-352490, Japanese Unexamined Patent Application (by PCT route) Publication No. 10-506479, SID 91 DIGEST pp. 762-765, 1991, and SID 92 DIGEST pp. 33-35, 1992).
In the liquid crystal display mode of the multi-domain vertical-alignment mode, however, the viewing angle cannot be increased satisfactorily, and the transmittance is sacrificed to increase the viewing angle. In particular, the protuberances and electrode slits formed on the surfaces of the substrates for the purpose of controlling the alignment do not serve as effective apertures, thus lowering the transmittance.
Accordingly, it is an object of the present invention to provide a liquid crystal display device of a multi-domain vertical-alignment mode which ensures a wide viewing angle without lowering the effective aperture ratio.
The present inventors found that it is effective, in achieving a wide viewing angle in a liquid crystal display device of a multi-domain vertical-alignment mode, to form a protuberance or an alignment disturbing surface serving as the alignment center on a counter substrate opposing pixel electrodes so that molecules of the liquid crystal are oriented in all directions around the alignment center when a voltage is applied. The present inventors also found that it is necessary, in order to prevent the effective aperture ratio from decreasing, to set the ratio of the area of the alignment center of the counter substrate (that is, the base area of a protuberance serving as the alignment center, or the surface area of an alignment disturbing surface serving as the alignment center) to the area of one pixel so as to be less than or equal to a predetermined ratio.
In order to achieve the above object, according to an aspect of the present invention, there is provided a liquid crystal display device of a multi-domain vertical-alignment mode including a driving substrate having a pixel electrode, a counter substrate opposing the driving substrate and having a counter electrode, and a liquid crystal sandwiched between the substrates, wherein molecules of the liquid crystal are aligned nearly perpendicularly to the substrates when no electric field is produced, and are aligned nearly horizontally by the application of a predetermined voltage, wherein the counter substrate has an alignment center portion for orienting the molecules of the liquid crystal in all directions centered on a certain point when the voltage is applied, and wherein the area of the alignment center portion of the counter substrate is set to be less than or equal to 5% of the area of one pixel.
According to another aspect of the present invention, there is provided a production method for a liquid crystal display device of a multi-domain vertical-alignment mode in which a driving substrate having a pixel electrode and a counter substrate having a counter electrode are opposed to each other, a liquid crystal is sandwiched between the substrates, and molecules of the liquid crystal are aligned nearly perpendicularly to the substrates when no electric field is produced, and are aligned nearly horizontally by the application of a predetermined voltage, wherein the counter substrate has an alignment center portion for orienting the molecules of the liquid crystal in all directions centered on a certain point when the voltage is applied, and wherein the area of the alignment center portion of the counter substrate is set to be less than or equal to 5% of the area of one pixel.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.